Terephthalate polyester resins like PET, CPET, PETG, PBT and PCT are thermoplastic polymers which are widely used in the plastics industry. PET and PETG are used in the manufacturing of film, bottles and plastic containers of all kinds. Virgin PET and PETG resins are approved for food packaging. PET is widely used in the manufacturing of bottles, for both carbonated soft-drink and alcoholic beverages. It is also used in the manufacturing of fabrics and textile materials. CPET and PCTA are used in the manufacturing of dual-ovenable food trays. Filled PET and PBT resins are used in the manufacturing of auto body panels, fenders, bumpers and grilles; relays, iron handles, lawn mower housings and the like.
A major problem with reprocessing terephthalate polyesters is that when heated in the presence of moisture, polyesters may partially hydrolyze to form some free glycol and terephthalic acid in the polymer melt. Polyesters must thus be very dry during processing, typically containing less than 50 ppm moisture, in order to prevent any hydrolysis. Inadvertently, these materials usually do partially hydrolyze during each processing cycle, hence limiting PET's use in the food packaging area to strictly virgin material alone.
With the growing use of terephthalate polyester products, major waste disposal problems and expenses have been encountered by both the manufacturers and the consumers. PET and CPET are currently the most widely used of all the terephthalate polyesters. These compounds are being used to produce consumer products such as bottles for carbonated soft-drink and alcoholic beverages, and dual-ovenable food trays. Typical scrap polyester products are shown in Table I.
TABLE I ______________________________________ Typical Scrap Polyester Products POLY- ESTER PRODUCT ______________________________________ PET Carbonated Soft-Drink bottle Beer, Wine or Liquor bottle Bottles for food products e.g. Mustard Packaging film (MYLAR .TM.) Photographic/X-ray film Fabric/Fibers (DACRON .TM., TERYLENE .TM.) Auto Fenders (BEXLOY .TM.) CPET Dual ovenable food trays Bottles for PETG Personal hygiene products e.g. Shampoo or Mouthwash Detergent Food products PBT Auto Bumpers Auto Grilles Auto Trim Distributor Caps PCTA Packaging film Dual ovenable food trays PCT High Temperature Engineered parts PCTG Sheet/Film for packaging ______________________________________
There are at present a number of avenues available to recycle or to degrade/depolymerize terephthalate polyesters. PET soft-drink bottles are currently being recycled, but with existing prior art processes, this material must be very clean, free of any contaminants such as bottle cap, paper or plastic labels, glue and high density polyethylene (HDPE) impact-resistant basecups, in order to provide a pure raw material which may be used for products such as carpet backing, pot scourers, fiber-fill and the like. The cleaning/segregation process is expensive and adds significant cost to scrap PET. If it is not cleaned, the contaminated PET may sometimes be only used in a low-value non-critical application. On the other hand, the clean PET may possibly be recycled several times. However, during each recycling/processing step, the polyester is further hydrolysed to the point where after several recycling "cycles" the polyester has degraded and shows a significant loss of mechanical properties, at which point it becomes an undesirable product.
When food is cooked in CPET dual-ovenable trays, the polymer is subjected to relatively long periods of moderate heat in the presence of substantial amounts of moisture released from the food. This thermal history as well as the hydrolysis caused by the moisture present during cooking may cause the CPET to degrade to the point where it shows significant loss of mechanical properties. Also, cooking food in the CPET may result in the staining and/or discoloration of the polymer. Hence the degradation and discoloration of CPET make it an undesirable material for recycling and there is at present no recycling effort being made for CPET.
These undesirable materials could either be used in a low-value non-critical application, such as fiber-fill in pillows and sleeping bags, or they could also be recycled by partially depolymerizing the high molecular weight polymer in the presence of a monomer to produce a low molecular weight polymer. These types of processes are disclosed in U.S. Pat. No. 3,037,048.
In U.S. Pat. No. 3,037,048, Lotz discloses a process through which scrap PET products such as filaments, films, fabrics and others are recycled to regenerate the compound dimethyl terephthalate (DMT). This process involves the depolymerization of PET and transesterification in the presence of methanol to yield DMT.
Further, U.S. Pat. No. 4,578,502 issued to Cudmore discloses the use of PET scrap for recovering ethylene glycol and either terephthalic acid or dimethyl terephthalate. The process includes depolymerization of a slurry of scrap PET by hydrolysis or methanolysis, and subsequently crystallization of the desired product. The disclosure of U.S. Pat. No. 4,578,502 is limited to the production of dimethyl terephthalate. In fact, at column 3, lines 22-28 of this document, it is stated that it is preferable not to use alcohols of a higher molecular weight than methanol.
It should be noted that in order to obtain a value-added product from scrap terephthalate polyester using the processes referred to in U.S. Pat. No. 3,037,048 (col. 2, line 64) and U.S. Pat. No. 4,578,502 (col. 3, lines 58-59), the use of extreme high-pressure reactors is required, hence involving a major capital investment for the set-up.
Hence, most of the PET/polyester recycling processes using transesterification presently known in the art are mainly aimed at producing starting materials that could be used for the synthesis of new terephthalate products. Therefore, it would be interesting to provide a one step process for the recycling of terephthalate polyesters that would yield products that could be readily used either in the plastics industry or otherwise. Plasticizers, lubricants, waxes and the like are good examples of these useful products of which plasticizers are readily usable products for the plastics industry.
Plasticizers are crucial constituents in the preparation of some plastic products. For example, dioctyl terephthalate (DOTP) is particularly useful to the PVC industry. Esters of this type can be synthesized mainly by reacting terephthalic acid with a suitable alcohol. Unfortunately in this case, the esterification reaction is very slow, thereby making the process economically undersirable. Alternatively, the transesterification of dimethyl terephthalate to DOTP through its reaction with 2-ethyl hexanol is an interesting reaction that yields DOTP rather quickly.
Therefore, suitable recycling alternatives that would yield a product of good commercial value would be highly desirable.